1. Field of the Invention
The invention relates generally to global positioning system devices and navigation receivers and more specifically to methods and apparatus for providing accurate position, velocity, and time solutions in high-acceleration, low-signal applications using only two receiver channels.
2. Description of the Prior Art
A unique direct-sequence spectrum-spreading code modulates each global positioning system (GPS) satellite signal by alternating the signal's phase by one hundred eighty degrees. The receiver commonly despreads the signal by multiplying it by a replica of the transmitted code. The despread signal is the sum of a component in phase with a real or hypothetical local-oscillator signal and of a second component ninety degrees out of phase with that local oscillator, which components together constitutes a two-dimensional signal vector whose angle corresponds to the despread signal's phase.
A data message carrying data that describe the satellites' positions and carry other information about the GPS system also modulates the signal by modulo-two addition with the spreading code at a fifty-hertz rate. Since the data message's content is not generally known in advance at the receiver, the receiver design commonly assumes the transmitted signal to be coherent for only the twenty milliseconds of each bit time.
In effect, the satellite's and the receiving antenna's relative motion further modulates the phase of the GPS signal, but in a continuous way rather than in steps of one hundred eighty degrees. The antenna's relative velocity therefore has the effect of a frequency modulation, or Doppler shift. The receiver commonly makes measurements of this motion-caused phase modulation, either as it affects the phase of the spreading-code modulation or the phase of the despread carrier or both.
The receiver commonly tracks the phase of the spreading code by inducing small code misalignments and measuring their effect in order to maximize the amplitude of the despread signal. The receiver also commonly measures or tracks the phase of the despread signal, whether or not those phase measurements also contribute directly to the navigation solution, in order to compute the Doppler shift, by manipulating the frequency or phase or both of the local-oscillator signal. The Doppler shift, which is the rate of change of the signal phase, must be known to tune the receiver, since the possible Doppler shift of many kilohertz is much greater than the narrow bandwidth needed to receive each despread GPS signal.
The receiver commonly averages over time to reduce the noise of phase measurements, either before or after the nonlinear phase-detection operation of measuring the signal vector's phase angle. Because of this operation's nonlinearity, the signal must dominate the noise at the phase detector if the receiver is to recover useful phase information from the signal. This effect, in which signal-to-noise ratios below a certain detection threshold at the antenna cause substantial loss of information, is common to angle-modulation systems, including common frequency-modulation broadcasting.
Detecting phase first in a wide bandwidth to create a phase function .phi.(t), then averaging over a time T which coincides with or is within the twenty-millisecond data-bit time during which the signal is coherent ##EQU1## has the advantage of eliminating the need to synchronize any pre-detection averaging or band-limiting with the bit edges, since the duration of the contamination of the phase function by the incoherence at each data-bit edge is a negligible fraction of the bit time. This technique is therefore often used when a high signal-to-noise ratio can be relied upon.
Averaging the signal vector first for as long a time as it is coherent, that is, for the twenty-millisecond bit time, ##EQU2## has the advantage of significantly lowering the detection threshold, for example, by a substantial thirteen decibels when the predetection averaging is lengthened from one millisecond to the full twenty milliseconds. This allows use of much noisier signals. Although the scaling shown is correct for the vector average, the scaling of this or any predetection averaging is actually irrelevant since phase detection discards amplitude information.
Except in special cases in which time, altitude, or other position information is known, a GPS navigation receiver must track at least four satellites to find its antenna's position in three dimensions. A receiver that has enough channels can dedicate a channel to tracking each satellite. When there are more satellites to track than channels to track them, receivers ordinarily resort to time-sharing strategies. Deliberately providing only a few channels to be shared among the satellites tracked reduces size, hardware complexity, and cost, at the expense of loss of signal power or risk of loss of carrier-phase lock.
A "multiplexing" receiver, as referred to herein, completes at least one cycle of channel-sharing during each twenty-millisecond bit time. It operates otherwise much like a multi-channel receiver, receiving phase and data almost continuously from all satellites tracked. However, a four-satellite multiplexing receiver averages the signal for no more than one-fourth of the bit time, and therefore pays a six-decibel detection-threshold penalty.
A "sequencing" receiver, as referred to herein, dwells on each satellite for one or more bit times. Sequencing receivers usually include two channels, one to sequence among the satellites for navigation and one to dwell on one satellite for a longer time to accumulate data, since the sequencing channel misses most of the data bits from any one satellite.
A sequencing receiver that dwells on each satellite for the one bit time needed for optimum linear processing is a "fast-sequencing" type, as used herein. Assuming that it can visit four satellites within one hundred milliseconds, the largest step that a nineteen meter-per-second-per-second acceleration can produce in that 100-millisecond cycle time is ninety-five millimeters or one-half wavelength at the L1 carrier frequency of 1.5754 gigahertz. Thus acceleration of 1.94 times that of earth's gravity, or 1.94 G, is needed before the phase uncertainty exceeds the maximum that can be identified unambiguously.
This does not mean however that it is possible to track carrier phase under two-G acceleration with a single GPS receiver channel. Even though the carrier phase can be determined in a single 100-millisecond interval, the Doppler shift due to velocity is still unknown. So, without a way to remove the effect of the present velocity from the next measurement, the phase-shift due to further acceleration will add to that due to present velocity. The quickest way to measure velocity from phase measurements every one hundred milliseconds is by differencing successive phase measurements to get the average velocity between them. The greatest velocity change that a 0.97 G or 9.5 meter-per-second-per-second acceleration can produce in the one hundred milliseconds between the centers between velocity measurements is 950 millimeters per second, which velocity corresponds to a phase change of ninety-five millimeters in the one hundred milliseconds between phase measurements or one-half wavelength at the L1 frequency. The practical result is to preclude tracking carrier phase per se across the gaps in each satellite's reception except for the most sedate applications with accelerations under one G.
Consequently, sequencing receivers have commonly been designed to reacquire each satellite for each new cycle of the sequence. Such a receiver ordinarily needs about five seconds to sequence a channel around to all the satellites being tracked. Thus it is a "slow-sequencing" type as used herein. The signal strength needed to acquire or reacquire a satellite is much greater than that needed to maintain continuous tracking. Thus the signals must be strong; so conventional slow-sequencing receivers, like multiplexing receivers, are particularly hampered by low signal levels. In summary, slow-sequencing receivers sequence slowly because they must reacquire the satellites; and they must reacquire the satellites because so much time elapses between successive measurements due to their slow sequencing.